Proceedings of the 1992 Linear Accelerator Conference, Ottawa, Ontario, Canada

EMITTANCE MEASUREMENT FOR HIGH-BRIGHTNESS ELECTRON GUNS

H. Kobayashi, Y. Yamazaki*, T. Kurihara, I. Sato, A. Asamit ,S. Otani**, Y. Ishizawa**
Photon Factory, National Laboratory for High Energy Physics, Oho, Tsukuba, Ibaraki, 305 Japan*The Graduate University for Advanced Studies, Oho, Tsukuba, Ibaraki, 305 Japan**National Institute for Research in Inorganic Materials, Namiki,Tsukuba, 305 Japan
Abstract
An emittance measurement system based on a highprecision pepper-pot technique has been developed for electronguns with low emittance of around 7tmm-mrad. Electron gunswith a 1 mm<J> cathode, the material of which is impregnatedtungsten or single-crystal lanthanum hexaboride (Lal-xCex)B6,
have been developed. The performance has been evaluated byputting stress on cathode roughness, which gives rise to anangular divergence, according to the precise emittancemeasurement system. A new type of cathode holder, which is a
modified version of the so-calIed Vogel type, was developed andthe beam uniformity has been improved.
Introduction
Both the generation of a small-emittance beam and its
acceleration without emittance growth (future themes at the
Test Linac 1) of the KEK Photon Factory) are crucial foradvanced applications of linear accelerators, such as a freeelectron laser. Emittance growth during transport andacceleration and its prevention have been the subjects ofextensive studies. 2)3) An electron gun with a smalI emittance,
which can be used as a reference beam generator, and a very
precise emittance monitor are the key components in suchstudies.
The thermal limit of the normalized rms emittance of acathode with a radius of rc at an absolute temperature of T isgiven by
En=27trc(kT/moc2)1/2(1)
A comparison between En and the measured value is ofinterest.
Lau has evaluated the emittance determined by the cathoderoughness regarding both temperature-limited and space-chargelimited emission. 4 ) Loschialpo et al. has measured theemillance of a large area polycrystal LaB6 cathode withprotrusions of two to three microns height, finding a greater
emittance than the thermal limit. 5) Few studies, as far as weknow, have been carried out concerning the roughness ofvarious cathodes.
A dispenser cathode with a long history has made great
progress during recent years. A current density of even
100A/cm 2 (but with a comparatively short lifetime) iscommercially available. A dispenser cathode with craters, but
no protrusions on the surface, is available. A single-crystal
lanthanum hexaboride(Lal_xCexB6) cathode with a high crystal
quality has recently been developed. 6) Part of the surface of thesingle crystal is extremely flat. These new cathode materials

TU4-17

have different surface conditions from that of poly crystal LaB6
and factors which give rise to an angular divergence havestimulated us to investigate electron guns with these materials
using a precise pepper-pot method.
Emittance Measurement Technique
Details concerning the emittance measurement system usedin these experiments wiIl be described elsewhere.?) A
schematic block diagram is given in Fig. 1, and outlines of themeasurement system are described. The pinholes of the pepperpot mask are 30 fJ.m in diameter and opened at intervals of 200
fJ.m radially or at intervals of 300 fJ.m with a grid pattern. A
plastic scintilIator with a fast temporal response and with ahigh spatial resolution due to the 10 fJ.m thinness is employed
to measure the images of beam lets cut out by such small
pinholes. A microscope used to observe small luminous 300to 700-fJ.m spots on the plastic scintillator and an image
intensifier with a high-speed shutter (minimum time of 3 ns)
to measure the beam images at the flat top of the beam pulse
and a CCD camera are mounted on a precise movable platform.
The magnified spot images can be measured one by one byscanning the optical system. A magnetic field, if desired, canbe applied between the pepper-pot and the scintiIlator toconfirm the energy of electrons comprising the spots. The
features of this emittance measurement system are summarizedin Table 1.

FaradaY-Clip~300mm -'vae. ,I\

retractable alU.ffi ina screen.
.
._
I c,
z-translator
Image mten"iliLer. I" : !

gun

"

~

,

lon~ distance.~, -.

I -microscope

SCintillator~::~1~~1~ anode pepper-pot \
\

.

\

~~\

CCD camera
.

'.

i

graphic analYJ'er

xyz-axis translator

Fig. l. Emittance measurement system

t Present address : Naruto University of Education,
Naruto-cho, Naruto-shi, Tokushima, 772, Japan.
341

Proceedings of the 1992 Linear Accelerator Conference, Ottawa, Ontario, Canada

(b)

(a)

TABLE-I. Features of Emittance Measurement System

0.1

Pepper-pot mask
Ti foil 20 f1 m thickness, pinhole 30 f1 m diameter

200 f1 m radially or 300 f1 m grid pattren
Scintillator
10 f1 m thickness, luminance peak 375nm,
rise time : 350ps, decay time: 1.6ns

Optical systemlong distance microscope(resolution 3 f1 m)image intensifier(minimum shutter time of 3ns)
mounted on xyz movable platform

2.0

I

1.5

'" 1.0osrrar..
o0

1.~5"2~.0"2~.50.'5• •
1.0• •0.5

Z[mmJ

In an electron gun a space between the heated cathode andthe wehnelt electrode at normal temperature is necessary forthermal insulation. The cathode is held in a so-called Vogeltype 8) holder; the vicinity of the cathode is shown in Fig.2(a)upper. At the beginning of the gun design we wanted to makethe space as small as possible, thus adopting the 0.1 mm, justone mesh size in a calculation in which the area between thecathode and the anode is divided into four. l ) The current-density
distribution measured by the pepper-pot system described above
was very different from that predicted by the EGUN code,9) andthe small space seemed to greatly deflect the beam trajectory.
Accordingly, the electron trajectories in the very vicinity of thecathode were calculated based on a small mesh size of 0.025
mm. When the cathode is held at the same position with thewehnelt electrode longitudinally (both surfaces in one plane)
only the beam from the edge of the cathode is deflected by theelectric field distorted by the space. The beam trajectories,
however, are greatly affected by a longitudinal cathode shift
with respect to the wehnelt electrode position. Only a 0.1 mm
shift gives rise to an angular divergence extending over theentire cathode; the beam trajectories and phase space of thebeam in the very vicinity of the cathode are shown in
Figs.2(a)-middle and -lower respectively. We developed amodified Vogel-type cathode holder in which the cathode issurrounded by a carbon heater so as to avoid space effects(Fig.2(b)-upper). The relative position of the cathode and thecarbon in this type can be easily adjusted to make one plane.
The influence of the space between the carbon and the wehnelt
electrode as well as the relative longitudinal position between
the carbon with the cathode and the wehnelt electrode becomes
negligibly small. Even if the cathode is attached behind thewehnelt electrode by 0.2 mm, the beam trajectories are notdistorted; Fig.2(b)-middle shows this situation with the phasespace of the beam in Fig.2(b)-lower. When the cathode withoutcarbon was attached behind the wehnelt electrode, two pinhole
images appeared through one pinhole. Typical pinhole images
at the beam edge without and with the carbon surrounding thecathode are shown in Figs. 3(a) and (b), respectively.

342

1.0

1.5

2.0

2.5

Z[mmJ

Cathode heating method

0.020

0"

-0.02

0""

'0 -0.04

E.

1 ::::'" -0.06

'"

-0.08

/"'eJ"oI"'"

0.005

-0.10
0.05

0.10

0.15

R[mmJ

0.20

O.OOO~O~..c.......-'-'::'0.::'::25~.w...........w.0.5:70..........JR[mmJ

Fig.2 Cathode holder and beam trajectory and phase space
without(a) and with(b) the surrounding carbon.
(a)

(b)

Fig. 3 Typical example of images at beam edge without(a) andwith(b) the surrounding carbon

Emittance of the Dispenser and LaB6 Cathodes
It is pointed out that the roughness of cathode surface is oneof the important conditions which determine the brightness.
Lau has evaluated the emittance determined by the cathoderoughness for both temperature-limited and space-charge-limited
emission, as described above. Distorted pepper-pot images due
to protrusions of cathodes were often observed in our
experiments. We thought that the surface(100) of the single

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Proceedings of the 1992 Linear Accelerator Conference, Ottawa, Ontario, Canada

crystal LaB6 is extremely flat compared with those ofpolycrystal LaB6 and of dispenser cathodes. This is of interest
from the viewpoint of cathode roughness. Emittancemeasurements of electron guns with the dispenser-type cathodeand with single-crystal LaB6 have been carried out.
The single-crystal (Lal-xCex)B6 prepared by the RFheating floating zone method 6) is mounted in the modified
Vogel-type cathode holder described above. In the case of
single-crystal LaB6, protrusions of several microns height tend
to remain after electro-discharge machining; a fresh surface after
machining and very large pepper-pot images at this roughness
are shown in figs.4(a) and (b), respectively. However, bykeeping the cathode at 1600 0C, the surface became graduallyflat due to evaporation, and the pepper-pot images became
small with the surface condition. Then, we prepared a polishedsingle-crystal LaB6 cathode to obtain a very flat surface. The
surface of the cathode after 72 hours use and the pepper-pot
images at this stage are shown in figs.5(a) and (b),respectively. A current of 496 rnA (density of 57.3A/cm 2 ) anda brightness of 5.7x10 10 Nm 2rad 2 (en=0.941tmm-mrad) wereobtained at 15800C.
(a)

(b)

several microns in size and are spread over the cathode.
According to very rough estimations by the EGUN code,
craters with a 6 ~m diameter and a 3 ~m depth give rise to anangular distribution equivalent to the cathode temperatureduring operation in this condition; however, the protrusions
with similar dimensions give a far larger angular velocity. The
cause of the very distorted images for a cathode without
polishing can be attributed to the protrusions. For polishedcathodes, a current of 412 rnA at the current density of
36.4A/cm 2 with a brightness of 3.5x10 10 A/m 2 rad 2(en= 1. I1tmm-mrad) was obtained at 1080°C. The emittancevalues for both the dispenser cathode and the single-crystal
LaB6 are almost the same in these experiments. The difference,
however, is that oval images are often observed for thedispenser cathode, in the contrast to circular images for the
single-crystal LaB6. Emittance of 95 percent beam has beenused and error of ±20 percents is contained for thesemeasurements.
Summary
A precise pepper-pot technique has been developed.
Electron guns with a dispenser and LaB6 cathode have beendeveloped and their beam emittances measured. When
protrusions of several microns in size are on the surface of thecathode, a clear spread of the pinhole image has been observed.
A dispenser cathode with craters of several microns in size andsingle-crystal LaB6 showed similar emittances in thesemeasurements, though the intensity distribution of the pinholeimage for LaB6 is somewhat different from that of dispenser
cathode and this is the subject for future study. Modified Vogeltype cathode holder improved beam uniformity.
References

t-1

lO~m

Fig. 4 Surface of single-crystal just after machining(a) andpepper-pOl images(b).
(a)

----<

(b)

lO~m

Fig. 5 Surface of polished single-crystal LaB 6(a) and pepperpot images(b).
We have prepared two different types of dispenser cathodes
with and without polishing after machining. In theseexperiments, cathodes coated with an iridium are used under thecondition of a temperature-limited region. Some cathodes
without polishing showed rough local regions; a greatly
distorted images were observed. In the case of dispenser
cathodes with polishing, the craters among tungsten-grains are

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